1. Field of the Invention
The present invention is generally related to a method for efficiently controlling access to a communication system and in particular to a method for preventing the occurrence of overload conditions in a communication system by controlling the amount of users having access to the communication system.
2. Description of the Related Art
Communication systems, and in particular, wireless communication systems comprise a plurality of communication channels through which subscribers of such systems communicate with each other and with the system. Wireless communication systems such as Code Division Multiple Access (CDMA) systems and other communication systems have a certain capacity; that is they are limited by the amount of communication channels that can be made available to subscribers of such systems. The capacity of a communication system is the amount of information per unit time (i.e., information rate, R) that can be conveyed (within the system) while maintaining an acceptable quality of communications. The acceptable quality of communications is typically defined by the operator of the communication system. Usually, the capacity is directly related to the number of subscribers using the system; the more subscribers using the system the higher the information rate.
Referring to FIG. 1 there is shown part of a typical topology of a cellular CDMA wireless communication system. The communication system depicted in FIG. 1 comprises a plurality of cells each of which delineates a particular geographical area or terrain that is covered by the communication system. The; cells have borders which form hexagons. The hexagons (108, 114, 110, 112) symbolically represent areas of coverage within which subscribers located in the same cell communicate with the cell (i.e., cell system equipment). Each cell has system equipment (owned and controlled by a system operator) that are used by the system to admit subscribers to the system; that is to allow subscribers of the system to gain access to the communication system for communicating with each other and/or with the system. At least part of the system equipment is typically located at a base station (e.g., 100, 102, 104, 106). Some of the system equipment at the base stations are Radio Frequency (RF) transmitters and receivers for conveying (i.e., transmitting and receiving) communication signals.
Other system equipment, which can also be located at a base station, provide the Operations, Administration and Maintenance (OAandM) services typically associated with communications equipment. For example, subscriber billing, allocating communication channels for subscribers, and giving subscribers access to the communication system are some of the services provided by the OAandM equipment. Subscribers given access to the communication system can communicate with other subscribers via the cell""s base station.
For a CDMA communication system, such as the one depicted in FIG. 1, a subscriber gains access by making a request to system equipment (usually located at a base station). For example, subscriber 124 in cell 112 makes a request to base station 100 to have access to the communication system. System equipment at base station 100 receive the request and decide whether to give subscriber 124 access to the communication system. Subscriber 124 and base station 100 (as well as other base stations and subscribers) communicate via communication channels called forward links and reverse links. The forward link is a communication channel through which base station 100 transmits communication signals to subscriber 124. The reverse link is a communication channel through which subscriber 124 transmits communication signals to base station 100. Thus, each subscriber has a forward link and a reverse link that it uses to communicate with system equipment and/or with other subscribers of the communication system.
Typically, the system decides to provide access to a subscriber by performing a power level analysis that attempts to maintain the quality of communications between subscribers at an acceptable level as defined by the system operator. The system could continue to admit subscribers requesting communication services (or responding to a system request or xe2x80x9cpagexe2x80x9d to admit subscribers) and thus keep increasing its information rate. At some point an overload condition will occur causing the quality of communications provided by the communication system to be degraded. An overload condition occurs when the quality of communications (e.g., existing voice and/or data calls) drops below an acceptable level set by the system operator. Usually the acceptable level is set as a threshold below the system""s ultimate capacity. One example of an overload condition is when a cell communicates with a relatively large number of subscribers such that the system cannot meet the desired signal to noise ratio (SNR) requirement. The number of subscribers that can be adequately serviced by a cell depends on the SNR usually expressed in terms of a ratio,             E      b              N      t        ,
where Eb represents the total received signal energy per unit of information (e.g., energy per bit) and Nt represents the total noise power density. The higher the       E    b        N    t  
of the signal measured at a receiver, the better the quality of communications.
The following equation defines the reverse-link signal to noise ratio       (                  E        b                    N        t              )        i    ,    k    m
for subscriber i in cell k as measured by cell m:                                           (                                          E                b                                            N                t                                      )                                i            ,            k                    m                ≡                                                            (                                  W                  R                                )                                            i                ,                k                                      xc3x97                          S                              i                ,                k                            m                                                          N              th                        +            J            +                                          ∑                                                      j                    =                    1                                                        j                    ≠                    i                                                                    M                  k                                            ⁢                                                υ                                      j                    ,                    k                                                  ⁢                                  S                                      j                    ,                    k                                    m                                                      +                                          ∑                                                      l                    =                    1                                                        l                    ≠                    k                                                  L                            ⁢                                                ∑                                      j                    =                    1                                                        M                    t                                                  ⁢                                                      υ                                          j                      ,                      l                                                        ⁢                                      S                                          j                      ,                      l                                        m                                                                                                          (        1        )            
The indices i and j designate particular subscribers and the indices k, l and m designate particular cells. Ml is the number of subscribers in cell l, Mk is the number of subscribers in cell k, L is the total number of cells in the system,       (          W      R        )        i    ,    k  
is called the processing gain for subscriber i in cell k where W is the bandwidth of a CDMA carrier signal and R is the data rate, as defined previously. The average power of the reverse-link signal is defined as the product of the power level (S) and the voice activity factor (v) of the reverse-link signal. In equation (1) above, Si,km, is the power level of the communication signal of subscriber i in cell k as measured at cell m, and the voice activity vi,k represents how often subscriber i in cell k speaks during a telephone conversation or how often a data energy burst is transmitted by the subscriber. Nth represents the power level of thermal noise typically generated in electrical and electronic circuitry. J represents the power level of any jammer signal, which is a type of interference. Any signal other than a subscriber""s communication signal is called interference. Two major sources of interference are thermal noise and jammer signals. For example, an external jammer interfering with a CDMA communication system is an analog mobile telephone user transmitting signals whose frequency spectrum is partially or entirely the same as the spectral band of the CDMA system; in such a situation the analog mobile signal interferes with CDMA subscriber signals. The aggregate power received by a base station is due to thermal noise, external jammers, and the CDMA subscriber signals. Thus, the total received power through a particular reverse link of a base station, which is called the received signal strength indicator (RSSI), has three components. The RSSI measured by cell m is defined by the following equation:                               RSSI          m                ≡                              N            th                    +          J          +                                    ∑                              l                =                1                            L                        ⁢                                          ∑                                  j                  =                  1                                                  M                  t                                            ⁢                                                υ                                      j                    ,                    l                                                  ⁢                                  S                                      j                    ,                    l                                    m                                                                                        (        2        )            
where the last term on the right-hand side represents the power of CDMA subscriber signals. Note that the RSSI increases when the number of subscribers increases, when the power due to jammer signals increases, and/or even when the thermal noise power increases.
A common and current practice in controlling system overload for the reverse link is to measure the increase in RSSI and decide, based on the RSSI rise, whether to admit to the system any subscriber requesting service. The measured RSSI is compared to a threshold and when this measured RSSI is substantially equal to or above the threshold, the system blocks additional access requests from subscribers. Such an RSSI based method assumes that a rise in RSSI is mainly due to the subscribers of the desired service.
Consequently, when the RSSI reaches a system operator defined threshold, no additional subscribers are admitted to the system. The flaw in this method is that many times a rise in RSSI due to external jammers is misinterpreted as being due to CDMA subscribers. Although an increase in RSSI due to external jammers should not affect the system""s capacity, such an increase in RSSI does indeed affect the system""s capacity when an RSSI based overload control method is used. A relatively strong jammer signal can trigger the overload condition prematurely reducing the system""s reverse-link capacity; this is an inefficient use of the communication channels of the system in that no additional subscribers are admitted even when the system can definitely handle such subscribers.
Another problem with the RSSI based overload control method is that many times certain subscribers who are currently using the system have to significantly increase their signal power resulting in an increase in RSSI thus potentially decreasing the       E    b        N    t  
for most of the other subscribers. The proper solution to this problem would be to identify and remove those particular subscribers from the system so as to reduce or eliminate the overload condition caused by such subscribers. However, the subscribers causing the rise in RSSI cannot be identified as their contribution to the RSSI cannot be separately identified and attributed specifically to them.
What is therefore needed is a method for substantially preventing overload conditions in a communication system based on the measurement of the signal power of the subscribers currently using the system and noise power which method is not adversely affected by the existence of external jammer signals. What is also needed is a method for substantially preventing overload conditions in a communication system by identifying and removing from the system specific subscribers currently using the system who are providing relatively large contributions to the overall interference power.
The present invention provides a method for substantially preventing overload conditions in a communication system based on a measurement of signal power and noise power called the call load which is associated with each of the subscribers using the system. The call load is independent of any interfering signals (e.g., external jammer signals). The contribution to the call load from each of the subscribers can be separately identified allowing a system operator and/or system equipment to alleviate overload conditions by removing from the system those subscribers deemed to be causing an overload.
In particular, the method of the present invention comprises establishing a threshold value for acceptable communications. Then, a call load analysis is performed from which an average call load value is calculated. When the average call load value is below the established threshold, communication channels are monitored for any subscribers requesting admission to the communication system and such subscribers are admitted. When the average call load value is substantially equal to or above the established threshold, subscriber admission requests are blocked and such subscribers are not admitted to the communication system. Admitted subscribers whose contributions to the call load are deemed significant or whose contributions tend to cause an overload condition are identified and removed from the communication system.